demandimport: make module ignores a set (API)
The list of modules to ignore is used for membership testing. Yet
it is defined as a list.
Sets are more efficient for membership testing. So this commit converts
the module list to a set.
Since we took an API hit, I renamed the variable to further clarify
the change.
This appears to reduce the CPU time for running 300 invocations of
`hg log -r. -T '{rev}'` on my i7-6700K:
before: 18.64s
after: 18.44s
Differential Revision: https://phab.mercurial-scm.org/D3440